How do the pigs manage to turn the misfortune into a victory? What does Napoleon reward him with? Does he deserve it?

Answers
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I'm not sure what you mean by misfortune in this chapter. The animals are getting older, some are looking to retire, and as usual Squealer is busy covering the "books" so it can be proven that everyone is well taken of.

On the other hand, Boxer is also seen being driven away in a van to his death, and the animals are able to read the sign that denotes where he's actually being taken;

Regarless of what they've seen, the animals are once again convinced by the pigs that it never happened, that Boxer dies in the hospital as Squealer cried next to his deathbed.

As for Napoleon's reward; he gave a short oration, a memorial banquet, and had a large memorial wreath made. He also brought in some whiskey. That's all I see in this chapter..... I'm not seeing a reward or misfortune of the pigs.